Full color printing has become a desired goal of office products. Digital printing machines typically provide operators with the ability to adjust print appearance before the start of a print run. For example, color adjustments allow operators to optimize print appearance of output copies.
Color printing is accomplished by providing multiple layers, or separations, of colorants, such as ink, toner, etc. on a page. For example, colors are commonly provided by combinations of cyan (C), magenta (M), yellow (Y), and black (K) inks. The CMYK process is in general use.
The process of color printing involves a significant number of complex variables which need to be monitored and adjusted to insure accurate color rendering. Process control for color printing involves the collection of data about the current color rendering characteristics of the printer. Calibration data is generally collected by generating electrostatic test patches which are traditionally printed in interdocument zones on the photoreceptor. These data are used to measure the deposition of toner on paper and result in tone reproduction curve or other expression of calibration reference data. The tone reproduction curve represents a correspondence of printer inputs to printer outputs. Several systems for measuring and estimating tone reproduction curves from color image data are described in U.S. Pat. Nos. 6,035,152 and 5,963,244 commonly owned by the assignee of this application and incorporated herein by reference. Higher levels of calibration are available in which parameters such as gray balance and cross separation are also calibrated.
A process control which uses tone reproduction curves is described in U. S. Pat. No. 5,777,656, commonly owned by the assignee of this application. The disclosure of the '656 patent is incorporated herein by reference. In the system described in the '656 patent, there is provided a method of maintaining tone reproduction in a printer, including the steps of generating a continuous tone image based on an original document. Through raster image processing the tone image is encoded by dividing the continuous tone image into a plurality of pixels and generating pixel data for each pixel. The pixel data has a position for each pixel and a desired darkness of gray of the corresponding pixel. Representative halftone targets or patches are generated on the photosensitive surface with toner. The amount of toner on each of the representative halftone targets is then sensed to generate a representative tone reproduction curve. A feedback signal is produced by comparing the representative tone reproduction curve to a setup calibration tone curve. The pixel data of each pixel of the final halftone image is adjusted to compensate for deviation between representative tone reproduction curve and the setup calibration tone curve during printer operation to achieve the desire tonality of the final halftone image.
The above control process is designed to maintain a consistent tonal quality to multiple copies or prints of the same image. The output, however, is determined by the tonal quality as set up by the calibration tone reproduction curve. Since modern color printers are capable of producing extremely complex and interesting images, there is a need to allow further adjustment of the output image color by the user. Some control processes allows an adjustment of the output image color according to predetermined adjustment curves and data or by partial adjustment of the calibration tone curve, but such schemes make it difficult to reproduce a particular output in subsequent processes. It is a purpose of this invention to provide a system for allowing the user to adjust the color independent of calibration. It is also a purpose of this invention to incorporate this capability into a system which also relies on color maintenance through the use of calibration tone reproduction curves or other similar mechanisms.